Universal Squash Model For Optical Communications Using Linear Optics And Threshold Detectors

TL;DR

This paper demonstrates that a universal squash model can be constructed for many quantum optical protocols, bridging the gap between theoretical single-photon models and experimental multi-photon implementations, despite previous no-go theorems.

Contribution

The authors show that relaxing certain measurement preservation conditions allows for a universal squash model applicable to various quantum protocols.

Findings

Universal squash model exists for many protocols.

The model applies to quantum key distribution, tomography, Bell tests, and entanglement verification.

It overcomes limitations imposed by previous no-go theorems.

Abstract

The transmission of photons through open-air or an optical fiber is an important primitive in quantum information processing. Theoretical description of such a transmission process often considers only a single photon as the information carrier and thus fails to accurately describe experimental optical implementations where any number of photons may enter a detector. It is important to bridge this big gap between experimental implementations and the theoretical description. One powerful method that emerges from recent efforts to achieve this goal is to consider a squash model that conceptually converts multi-photon states to single-photon states, thereby justifying the equivalence between theory and experiments. However, up to now, only a limited number of protocols admit a squash model; furthermore, a no-go theorem has been proven which appears to rule out the existence of a universal squash model. Here, we observe that an apparently necessary condition demanded by all existing squash models to preserve measurement statistics is too stringent a requirement for many protocols. By chopping this requirement, we show that rather surprisingly, a universal squash model actually exists for a wide range of protocols including quantum key distribution protocols, quantum state tomography, the testing of Bell's inequalities, and entanglement verification, despite the standard no-go theorem.